In modern telecommunication networks—in particular digital fixed networks such as ISDN, or also in the mobile radio standards GSM (Global System for Mobile Communication) and UMTS (Universal Mobile Telecommunication System)—the Signaling System No. 7 (also known as CCS7=Common Channel Signaling System 7) is used. This refers to a modular digital signaling system which is based on the separate transmission of user information and control information. This gives rise to a network exclusively for signaling information which is separated not only logically, but also physically from the lines for user data transmission.
A fundamental concept of this signaling system consists in the subdivision of the CCS7 functions into MTPs (Message Transfer Parts) which are consistent for all applications in the bottom three of the four protocol layers, and application-specific UPs (User Parts) on the top layer. (“User”, in this context, is understood to mean not a subscriber, but an entity using the signaling system in the network.)
To allow the use of services and service features in ISDN which result from the integration of telephone and data services, different variants of what are termed ISUPs (ISDN User Parts) have been created. With ISUPs, it is mostly data for the setup and cleardown and for the monitoring of circuit-switched B-channel connections which is transmitted between the network nodes in the ISDN. The ISUPs are structured with certain specific characteristics at national level within the framework of the standardization specifications published by the ITU. In parallel with this, international ISUPs, as they are called, have been created for the signaling of international telecommunication connections.
In traditional ISUP networks, the interworking between the ISUP variants for international connections is implemented in an international gateway. For international calls, the outgoing international gateway converts the national ISUP into the ITU-T-compliant international ISUP. In the destination network, this international ISUP is converted once again into a national ISUP in the incoming international gateway.
For some time now voice transmission via IP-based data networks (VoIP=Voice over IP), in particular the Internet, has attracted increased attention as a special telecommunication method. Whereas originally considerable cost savings had been expected and had stimulated the development efforts aimed at implementing suitable interfaces and other system components, today the advantageous opportunities for using added value services take center stage.
In connection with the development of VoIP, convenient services are increasingly being implemented and made available, such as, for example, videoconferences comprising a number of subscribers (multicasting). For this, SIP (Session Initiation Protocol) is available as an internationally standardized signaling protocol for setting up a call between subscribers. In contrast to fixed network subscribers, the location of a SIP subscriber is not tied to a specific line; rather, it is not restricted geographically thanks to a flexible registration scheme. Therefore, the main function of SIP is to discover the current location of the called subscriber. With the call setup (session setup), SIP transports a session description which allows the subscribers to negotiate the type of communication (e.g. audio or video) for this session.
SIP-T (i.e. SIP for telephones) is a SIP protocol extension enabling transparent transport of ISUP signaling information over a VoIP network. SIP-T thus guarantees feature transparency for subscribers that are connected to PSTN/ISDN exchanges.
However, the implementation of SIP-T requires that the ISUP protocol variant is identical at the entry to and exit from the VoIP network, because no conversion of ISUP protocol elements can be performed in the VoIP network. Thus, if a network provider A wants to handle both internal network traffic and external traffic to a network provider B via the VoIP network, an interworking problem results because the ISUP variants in the networks A and B are different in the majority of cases.